Tuning optical properties in the long wave infrared is typically the domain of semiconductor materials where plasmon-polariton or coupled plasmon-phonon-polariton responses can be modulated by altering carrier concentrations. Less explored is the use of other tunable material-photon interactions to alter optical properties. This is because optical responses in the infrared are typically considered invariable within a specific dielectric material.
Optical phonon energies of many semiconductors are commensurate with the energies of photons within the infrared. Therefore, infrared (IR) photons can couple with near-zero wavevector optical phonons resulting in a band of high reflectivity, commonly denoted the Reststrahlen band, in partially ionic materials. In this band, the real portion of the electric permittivity crosses zero and can become strongly negative. Since the permittivity at these frequencies is described based upon the energies and scattering times of the transverse and longitudinal optical phonons, variations in the phonon response will change the optical response. Previously, it has been shown that limiting the mean free path of optical phonons in an ionic material, magnesium oxide, can change the infrared optical response of the material by affecting phonon-polaritons. In particular, a decrease in crystalline coherence from 47 nm to 5 nm resulted in a 20% reduction in the reflectivity of the material in the IR Reststrahlen band. This was due to a change in the IR dielectric properties near the phonon-polariton resonance arising from variations in the scattering time of the optical phonons. See J. F. Ihlefeld et al., Appl. Phys. Lett. 97(19), 191913 (2010).